1. Field of the Invention
The present invention relates to add/drop multiplexer apparatus for use, for example, in synchronous digital hierarchy (SDH) communications networks to connect tributary channels of the network to main channels thereof.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows one example of an STM-4 add/drop multiplexer (ADM) previously considered for use in an SDH network. An optical transmission line 2, for example an optical ring, carrying a synchronous transport module at level 2 of the SDH hierarchy (STM-4), passes through the ADM 1. The bit rate of the STM-4 module is 622.08 Mbit/s (hereinafter 622 Mbit/s).
Each STM-4 module carries four SDH level 1 (STM-1) modules in time-division multiplexed form. These four STM-1 modules can be regarded as providing the transmission line 2 with four separate 155.52 Mbit/s (hereinafter 155 Mbit/s) main channels extending in parallel. Each STM-1 module (main channel) can be used to transport a plurality of individual lower-bit-rate information signals (tributary signals), and FIG. 2 of the accompanying drawings shows one example of the structure of an STM-1 module used to transport up to 63 2.048 Mbit/s (hereinafter 2 Mbit/s) tributary signals.
As shown in FIG. 2, each 2 Mbit/s tributary signal to be transported within an STM-1 module is incorporated in a container C-12. A path overhead (POH) is added to the container to form a virtual container VC-12. This path overhead (POH) contains information identifying the tributary signal concerned, error checking information, and information identifying the type of container (in this case 2 Mbit/s) involved.
Each virtual container VC-12, has, associated therewith, a pointer indicating the start point of the virtual container VC-12 concerned in its STM-1 frame, the virtual container and its pointer together forming a tributary unit TU-12. Thus, the containers C-12 are the respective payloads of the tributary units TU-12.
In FIG. 2, the 63 tributary units TU-12 which can be accommodated within an STM-1 module are combined in groups of 3 to form twenty-one tributary-unit groups TUG-2. These groups are then further combined, by bit interleaving and addition of path overheads (POH), to form a higher-order virtual container VC-4 which is located within the module by means of a further pointer. The higher-order virtual container VC-4 and its associated pointer constitute an administrative unit (AU). In the example shown in FIG. 2, the administrative unit AU-4 forms the entire payload of the module STM-1, the module only further including section overhead (SOH) required for transmission purposes.
Further information on SDH networks can be found, for example, in "The New CCITT Synchronous Digital Hierarchy: Introduction and Overview", Harrison K. R., British Telecommunications Engineering, Vol. 10, July 1991, page 104, and in ITU-T Recommendations G.707 (Synchronous Digital Hierarchy Bit Rates), G.708 (Network Node Interfaces for the Synchronous Digital Hierarchy) and G.709 (Synchronous Multiplexing Structure).
Returning now to FIG. 1, in the ADM 1 first and second VC-4 time slot assignment units (TSA) 3a and 3b are provided at the interfaces of the ADM with respective left-hand and right-hand portions 2a and 2b of the optical transmission line 2. These TSAs 3a and 3b are multiplexers/demultiplexers (muldems) and signal distributors which serve to provide access to the four higher-order virtual containers VC-4 in an STM-4 module; thus, the TSA 3a serves to provide access to the four VC-4s in the STM-4 module carried by the left-hand transmission line portion 2a, and the TSA 3b serves to provide access to the four VC-4s in the right-hand transmission line portion 2b.
The ADM 1 is also connected, via respective interface units 5a, 5b and 5c thereof, to three tributary channels 6a, 6b and 6c, each of which can carry an STM-1 module or a single 140 Mbit/s, 45 Mbit/s or 34 Mbit/s tributary signal or up to twenty-one 2 Mbit/s tributary signals.
The ADM 1 further includes a time slot interchange unit (TSI) 10 which has six ports P.sub.1 to P.sub.6. The first port P.sub.1 of the TSI 10 is connected via the TSA 3a to access a first higher-order virtual container VC-4#1 of the transmission line portion 2a. The second and third ports P.sub.2 and P.sub.3 of the TSI 10 are connected via the interface units 5b and 5c to access respectively the tributary signals of the tributary channels 6b and 6c. The fourth port P.sub.4 of the TSI is unused in this example. The fifth and sixth ports P.sub.5 and P.sub.6 are connected via the TSA 3b to access respectively first and second higher-order virtual containers VC-4#1 and VC-4#2 of the transmission line portion 2b.
The interface unit 5a is connected to access a second higher-order virtual container VC-4#2 of the transmission line portion 2a, and the third and fourth higher-order virtual containers VC-4#3 and VC-4#4 of the transmission line portion 2a are connected respectively to the corresponding third and fourth higher-order virtual containers VC-4#3 and VC-4#4 of the transmission line portion 2b.
The TSI 10 is essentially a patch panel provided between two multiplexers/demultiplexers, and in use of the ADM 1 serves to permit any virtual container in a channel connected to one of its ports to be added to or dropped from another channel connected to another of its ports.
Ideally, in the example shown in FIG. 1, the TSI 10 would enable any VC-12 in any of the tributary channels to be added to/dropped from any of the VC-4s in the transmission line 2. However, this would require the TSI to have access to all four VC-4s in each transmission line portion 2a or 2b, and also access to all the tributary signals. In turn, this would require the TSI to have eleven ports, and, because each VC-4 can contain up to sixty-three VC-12s, a capability of interchanging 693 (=11.times.63) VC-12s. However, the number of electronic circuitry gates required to provide a time slot interchange unit having such a virtual container interchange capability is prohibitively high and, even if technically feasible, the cost cannot always be justified at each tributary connection node (add/drop location) of a network.
Presently, a TSI having six ports and a maximum virtual container interchange capability of 378 (=6.times.63) VC-12s is contemplated. This falls far short of the desired virtual container interchange capability mentioned above and leads to system design limitations such as, for example in FIG. 1, the inability to add VC-12s of the tributary channels 6b and 6c to the third and fourth VC-4s VC-4#3 and VC-4#4 of either transmission line portion 2a or 2b. In addition, there is no continuity between the second VC-4 of the transmission line portion 2a and the second VC-4 of the transmission line portion 2b so that effectively one of the four main channels of the transmission line 2 is broken at the ADM 1; in FIG. 1 all VC-12s of the tributary channel 6a are passed by the TSA 3a to the second VC-4 of the transmission line portion 2a.
In view of the limited capabilities of present time slot interchange units, alternative ways have been proposed of realising a 2-622 Mbit/s ADM capable of adding/dropping 2 Mbit/s signals to/from any of the VC-4s in an STM-4 module.
In one such proposal, illustrated in FIG. 3 of the accompanying drawings, access to the individual 2 Mbit/s signals in all four VC-4s in each transmission line portion 2a or 2b is achieved by providing a 155-622 Mbit/s add/drop line terminal 50 and four 2-155 Mbit/s add/drop line terminals 60 at each transmission line interface. The proposal also uses a 2 Mbit/s patch panel 70 having ports for connection to all the accessed 2 Mbit/s signals of the VC-4s of the two transmission line portions 2a and 2b, and also has further ports for adding/dropping 2 Mbit/s tributary signals.
This arrangement is disadvantageous, however, since it uses a relatively large amount of hardware. Also, because it makes use of STM-4 and STM-1 line terminal equipment, the SDH signals are terminated at the 2 Mbit/s interfaces between the add/drop line terminals 60 and the patch panel 70 so that end-to-end path monitoring capability is lost.
A further proposal, shown in FIG. 4 of the accompanying drawings, uses a VC-4 time slot assignment unit 80 and a VC-12 time slot assignment unit 90 at each transmission line interface to provide access to all the VC-12s carried by the STM-4 modules in each transmission line portion 2a or 2b. A VC-12 cross-connect unit 100 has ports connected to all the accessed VC-12s and also further ports for adding/dropping VC-12s to/from tributary channels. The disadvantage of this proposal is that the switch matrix required is complex and requires sophisticated control.